In the various operations which take place in a wellbore--drilling, completion, fracturing, workwover, etc., it sometimes happens that a string of pipe or tubing which is supposed to be freely pullable becomes tightly stuck, and all efforts to free it by lifting and attempting to rotate it fail. In such event, it is typically necessary to sever the pipe at the free point, so that the free portion thereabove may be removed and salvaged. In order to salvage as much of the length as possible, the operator needs a rather precise idea of such length, which of course is the depth of the free point.
Heretofore there have been relatively few devices available to detect the free point of a string which is stuck in the hole. The best known such device, marketed by the DiaLog Tubular Survey Co., is described in U.S. Pat. No. 3,004,427, issued in 1961 to T. L. Berry. The Berry apparatus includes a downhole tool whose main element is a variable inductor, made variable by the fact that it has a 2-part iron core. The tool is set in place by belly springs above and below the inductor, the belly springs securing the entire tool to the inner wall of the string. When first set in position, the core parts are in a closed position which furnishes a particular reading to a galvanometer in the electrically connected surface equipment. The string is then stressed by an upward pull or a torque, and if the tool is located in the free part of the string the upper core half pulls away from the lower core half, changing the inductance and giving a different reading in the surface instrument. Since this depth is thus known to be free, the operator then lowers the tool and repeats the same steps at progressively deeper positions until he reaches one where the core parts do not separate. He then knows that his tool is in the stuck area.
While the Berry tool has worked admirably, it is an intricate and expensive tool. It has little in common with the present invention other than making use of the fact that the free part of the string will yield slightly under a tensile load, while the stuck portion will not.